I'd like to share some information that I read from Joe D'Aleo and Tom Downs yesterday, and from another Joe D'Aleo post this morning. The discussion was about the EPO or Eastern Pacific Oscillation. I'll have some other post in the next few days about some of the other important teleconnections that we'll be looking at this winter.

Here's the definition of the EPO:"A dipole pattern similar to the NAO in the Atlantic, but located in the eastern Pacific. There is a tendency for heights/pressures/temperatures to be higher to the north and lower to the south in the negative phase and lower to the north and higher to the south in the positive phase. The negative phase corresponds to widespread cooling over central and eastern North America and the positive phase to warming. - Source: WeatherBell Analytics."

​This pattern will become very important during winter because the negative phase of the pattern opens the door to very cold Arctic and Siberian air into the US. Here's an image that depicts the negative phase of the EPO.

Notice in this configuration how you have high pressure over low pressure off the west coast of the US. The next time you're looking at computer models, make a note of what the pressures are in the east Pacific, and especially look for this pattern this winter.

The weather pattern in the positive phase of this teleconnection is just the opposite of the negative (of course!), and in this case, you have low pressure over high pressure which gives the central and eastern US a different outcome.​

To give you an example of how the EPO effects temperatures across the US, take a look at October and January temperatures when we have a negative EPO. Keep in mind several things about teleconnections:

The effects of a teleconnection varies according to the time of the year you are looking at. A teleconnection that may have a large influence on the weather in the winter time may not have much effect at all during or times of the year.

You can't base a forecast off of one teleconnection. First, there are some teleconnections that may carry more weight than others, and all the teleconnections influence the others to some degree. It's a very complex inter-relationship that is constantly evolving in placement and strength.

For us here in the SE, there are a couple of other teleconnections that are much more important... the North Atlantic oscillation (NAO) and the Arctic Oscillation (AO). Some post about those two coming up.

Temperatures during negative EPO's in October (Image from http://madusweather.com/)

Temperatures during negative EPO's in January (Image from http://madusweather.com/)

Lately we've been running a positive EPO with the big low pressure over the Gulf of Alaska. The models are depicting a possible change over the next several weeks to a more negative phase, and that would most likely mean cooler temperatures than what we've had recently. Here's a look at the GFS ensembles and what it thinks the EPO will do through the end of the month. The Euro is actually more bullish. All of the models have been flipping around from run to run on this, and often that is a sign that a pattern change is in the air.

Notice that the EPO has been running positive lately and also notice that our temperatures have been above normal. The maps below show how October temperatures respond with a positive, neutral, and negative EPO. Again, the EPO isn't the only teleconnection responsible, but you can start to see how they do influence the weather. The three maps below are from Madweather.com

Good Saturday morning and the first day of October! ​The only real weather story and big question, is where will hurricane Matthew go. Let's first take a look at how Matthew did overnight and and then look at what the models are saying could happen.

Matthew went from a tropical storm to a CAT 5 hurricane in just 33 hours, a feat that no computer guidance and no forecast called for. It just goes to show how unpredictable tropical systems can be, and despite how far we've come in predicting hurricanes and their track and intensity, there are times that Mother Nature says "I've got this..." and proceeds to defy humans and all of their skills. You can't help but be humbled and awed by the sheer power and unpredictability of a beast like this. It's one of those moments that makes weather so awesome...

Credit for many of these images goes to WeatherBELL.

Overnight

Here's the 5 AM NHC discussion as to the state of Matthew:

Matthew has changed little in organization since the past advisory, with a small eye surrounded by a central dense overcast featuring cloud tops colder than -80ºC (-122ºF). The eye has become a little less distinct, suggesting at least that the hurricane is no longer intensifying. In addition, the raw intensity estimates from the CIMSS ADT technique are slightly lower than they were six hours ago. Based on these trends, the initial intensity is reduced to 135 kt (155 mph). An Air Force Reserve Hurricane Hunter aircraft is scheduled to reach Matthew around 1200 UTC (8AM).

The initial motion is now 270/6 (West @6mph). Matthew remains south of a low- to mid-level ridge over the western Atlantic. The dynamical models forecast this ridge to weaken over the next 72 hours as a mid- to upper-level trough develops over the Gulf of Mexico. This evolution should cause Matthew to turn northwestward after 24 hours and northward by 48-72 hours.

So Where To Now?

So... let's look at the official forecast and go from there.

The NHC is thinking that Matthew will brush the coast of Jamaica and cross the eastern end of Cuba before striking the Bahamas early next week. The thought also is that Matthew will lose some steam as it crosses Cuba.

"Matthew should experience significant shear for the next 24 hours or so, as water vapor imagery shows westerly flow blowing into the hurricane underneath the impressive outflow pattern. This should cause some weakening. After that time, the dynamical models suggest the shear should decrease, which should allow Matthew to at least maintain its intensity. The new intensity forecast shows a slightly weaker intensity than the previous advisory through 72 hours based mainly on current trends. Subsequently, the hurricane is likely to interact with the land masses of Jamaica, Cuba, and Hispaniola, leading to some weakening and disruption of the structure. Between this and uncertainty about how much shear Matthew will encounter north of Cuba, the new forecast shows only modest strengthening after Matthew reaches the Atlantic north of Cuba. There is also the possibility of fluctuations in intensity caused by eyewall replacement cycles at any time during the forecast period.

​ It is important to remind users that average NHC track forecast errors are around 175 miles at day 4 and 230 miles at day 5. Therefore, it is too soon to rule out possible hurricane impacts from Matthew in Florida."

So that is the official thought, now let's look at the computer models and compare the differences.

Computer Models Are Just Guidance...

There are some significant differences between the two major computer models, the American GFS and the European (or Euro or ECMWF). The GFS began a trend yesterday by shifting its tracks west toward the coast, while the Euro has gone much further east with its tracks. Here's a look at the Euro ensemble tracks (left) and the GEFS tracks to the right. Keep in mind that the Euro has 51 total members compared to the 21 members the GEFS has, and that is why the Euro looks more densely packed. It is however, the GFS members that are more tightly clustered from start to finish.

The Euro operational (and the majority of its members) takes Matthew right between Cuba and Hispaniola before crossing the southern Bahamas and out to sea. Minimal direct hits to land and then no worries. The GFS scenario is a much more dangerous option.

The GFS operational run mimics the ensembles and brings the hurricane right up the eastern US coast in what would be the most disastrous path that a hurricane could possibly take. Last night Joe Bastardi said the the only hurricane to do that was Hurricane Donna in 1960. I've included the GFS maps every 6 hours so you can see for yourself how bad this potential scenario would be to a densely populated and very vulnerable area. These are 850 mb wind speed and MSLP. (All images WeatherBELL)

And the bad part is that nothing has changed this morning, these are from the latest model run.

Wave Action

To really show why this would be so bad for the coast, take a look at the wave height and direction. The model shows 40 foot waves just off shore, with the waves to the north of the hurricane moving almost perpendicular to the shore, in other words, moving in a straight line toward the beaches. This would create a tremendous storm surge that would travel up the entire east coast. The forecast only goes out this far, but the track would continue to take it right along the shoreline all the way to Maine.

Other Models

Here's a look at a few of the other models with out comment.

The main reason for the big differences is the timing and the strength of the trough over the northern US. That trough, in the right location, will pull the hurricane toward it. If the trough is weaker, the hurricane is not pulled east as much, or if the timing is off and the trough misses the hurricane... lots of scenarios and no concrete answers yet. By tomorrow we'll know which model, the Euro or GFS, was correct.